The broad, long term goal of the proposed study is to determine, at the molecular level, the mechanisms by which DNA replication occurs in vivo. Faithful duplication of the genome is an essential step in the cell cycle. Any disturbance in this process can lead to cell death, genomic instability, or unregulated cell growth. Therefore, studying the mechanisms of DNA replication will contribute not only to broad areas of basic biology, but also to understanding the molecular basis for human diseases, such as cancer and developmental disorders. DNA replication requires highly coordinated recruitment of replication factors to replication origins in a chromatin context, as well as progression of DNA polymerases through a chromatin template. It is therefore expected that histone modification patterns around replication origins strongly affect DNA replication. It is highly likely that proper histone modification patterns are established around replication origins before each DNA replication cycle. However, how histone modification patterns change around DNA replication origins during cell cycle progression and how specific patterns of histone modification affect DNA replication have not been determined, due to technical limitations. We have recently developed a new system to purify an autonomously replicating chromatin circle in microgram quantities. Combined with an ultra-sensitive mass spectrometry instrument, our system gives us a unique opportunity to identify histone modifications in an unbiased fashion around DNA replication origins during cell cycle progression. We will use our system to test our hypothesis that histone modifications are under dynamic regulation during cell cycle progression and play critical roles in DNA replication.